1. Field of the Invention
The present invention relates to a magnetoresistance effect element, magnetic random access memory, magnetic head, and magnetic reproducer.
2. Description of the Related Art
Examples of the magnetoresistance effect elements include ferromagnetic tunnel junction elements such as a ferromagnetic single tunnel junction element and a ferromagnetic double tunnel junction element (or a tunneling magneto-resistance element “TMR element”). The ferromagnetic single tunnel junction element has a structure in which a pinned ferromagnetic layer, tunnel barrier layer, and free ferromagnetic layer are laminated. The ferromagnetic double tunnel junction element has a structure in which a pinned ferromagnetic layer, tunnel barrier layer, free ferromagnetic layer, tunnel barrier layer, and pinned ferromagnetic layer are laminated. Additionally, the “free ferromagnetic layer” is a ferromagnetic layer whose direction of magnetization is changed on application of an external magnetic field, and the “pinned ferromagnetic layer” is a ferromagnetic layer which maintains the direction of magnetization fixed on the application of the external magnetic field.
The magnetoresistance effect element can be used in a magnetic random access memory (hereinafter referred to as MRAM), magnetic sensor, magnetic head, magnetic reproducer, and the like. In the use, the magnetoresistance effect element is requested to have a sufficiently large magneto-resistance ratio (hereinafter referred to as MR ratio).
An MR ratio of 20% or more is obtained in the TMR element. For example, there has been proposed a ferromagnetic single tunnel junction element obtained by forming a thin Al layer having a thickness of 0.4 nm to 2.0 nm on a ferromagnetic layer, exposing the surface of the layer with an oxygen glow discharge or an oxygen radical, forming a tunnel barrier layer of AlOx, and further forming a ferromagnetic layer on the tunnel barrier layer. According to the ferromagnetic tunnel junction element, an MR ratio of 20% or more is obtained (J. Appl. Phys. 79, 4724 (1996)). Moreover, even in a ferromagnetic tunnel junction formed via magnetic particles dispersed in a dielectric, or a ferromagnetic double tunnel junction (continuous film), the MR ratio of 20% or more is obtained (Jpn. Pat. Appl. KOKAI Publication No. 1997-260743, Phys. Rev. B 56 (10), R5747 (1997)., Applied Magnetics Journal 23, 4-2, (1999), Appl. Phys. Lett. 73(19), 2829 (1998)).
The MR ratio of 20% or more is obtained in the TMR element in this manner. However, when a voltage to be applied to the TMR element is increased to obtain a sufficient output voltage, the MR ratio remarkably drops. The drop of the MR ratio is not large in the ferromagnetic double tunnel junction element as compared with the ferromagnetic single tunnel junction element, but the ratio is not necessarily suppressed sufficiently. Particularly, in a large-capacity (e.g., 256 Mbits or more in the MRAM) MRAM, hard disk drive, and the like, a larger MR ratio is required for the TMR element. That is, there is a strong demand for suppression of the drop of the MR ratio in the use.
Additionally, in Jpn. Pat. Appln. KOKAI Publication No. 2000-25123, there is disclosed a ferromagnetic double tunnel junction element structured by connecting a pair of ferromagnetic tunnel junctions having asymmetric voltage-resistance properties in series so that the voltage-resistance properties are symmetric with respect to a voltage application direction. Additionally, “the voltage-resistance properties are asymmetric” means that the voltage-resistance property in applying the voltage in one direction is different from the voltage-resistance property in applying the voltage in a reverse direction. In the ferromagnetic double tunnel junction element, the voltage-resistance properties of the respective ferromagnetic tunnel junctions constituting the ferromagnetic double tunnel junction are asymmetric, but the voltage-resistance property of the ferromagnetic double tunnel junction is symmetric. According to the ferromagnetic double tunnel junction element, the MR ratio can be set to be substantially constant within a range of about ±0.2 V.
Moreover, in the Jpn. Pat. Appln. KOKAI Publication No. 2000-25123, the following method is described as the method for realizing the asymmetric voltage-resistance property with the ferromagnetic tunnel junction.
A first method comprises: setting a composition to be asymmetric in the tunnel barrier layer of the ferromagnetic tunnel junction. Concretely, when the surface of an Al film deposited on a first ferromagnetic layer is oxidized to form the tunnel barrier layer, oxidation of the Al film proceeds from the surface of the film. This is used, and an oxygen concentration gradient is formed in the tunnel barrier layer. Alternatively, two types of sputtering targets are used in forming the film of the tunnel barrier layer, and a ratio of a power supplied to one target to a power supplied to the other target is changed with an elapse of time, so that a composition distribution is generated in the tunnel barrier layer.
A second method comprises: allowing an interface state to differ between a pinned layer side and a tunnel barrier layer side of the tunnel barrier layer. Concretely, the surface of the Al film deposited on the first ferromagnetic layer is oxidized and the tunnel barrier layer is formed. The oxidization is performed in such a manner that the ferromagnetic layer side of the Al film is prevented from being oxidized. After a second ferromagnetic layer is further deposited on the Al film having the surface oxidized, and annealed, atoms included in the first ferromagnetic layer are diffused in a non-oxidized portion of the Al film, and a layer of a solid solution of a material of the tunnel barrier layer and a material of the first ferromagnetic layer is formed.
It is remarkably difficult to control the composition distribution in the tunnel barrier layer with a high precision or to perform the annealing and control the interface state of the tunnel barrier layer with the high precision. Therefore, in the method described in the Jpn. Pat. Appln. KOKAI Publication No. 2000-25123, when a large number of ferromagnetic tunnel junctions are formed on one substrate, a large dispersion is easily generated in the voltage-resistance property within the substrate.